Al2 O3 Based ceramics

ABSTRACT

There is disclosed an Al 2  O 3  based ceramic which includes 5% to 50% by weight of a hard dispersed phase of at least one compound selected from metal carbide, metal nitride, metal carbonitride, metal oxy-carbide, metal oxy-nitride and metal carbo-oxy-nitride. The compound is represented by M(CNO), wherein M is at least one metal selected from the group consisting of Ti, Zr and Hf. The ceramic contains no greater than 25% by weight of ZrO 2 , and balance Al 2  O 3  matrix. which contains an additive dissolved in the Al 2  O 3  grains so as to form solid solution therewith. The additive is contained in an amount of 0.01% to 12% by weight with respect to the Al 2  O 3  matrix and is at least one oxide of an element selected from the group consisting of Y, Mg, Cr, Ni, Co, and rare earth elements. The Al 2  O 3  grains have an average grain size no larger than 1.0 μm while the hard phase constituent has an average grain size no larger than 0.6 μm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to an aluminum oxide (Al₂ O₃) based ceramic which has great strength and toughness, and exhibits excellent performance when used in cutting tools under severe conditions.

2. Prior Art

Japanese Patent Application laid open with Publication No. 53-118410 describes a conventional Al₂ O₃ based ceramic which is manufactured by the following method. First, Al₂ O₃ powder, powders of metal oxides and powders of hard dispersed phase constituents are prepared as starting powders. The metal oxides serving as the binder phase constituents are selected from those of yttrium (Y), magnesium (Mg), chromium (Cr), cobalt (Co), nickel (Ni) and rare earth elements (abbreviated to R), while the hard dispersed phase constituents are carbides (MC), nitrides (MN), carbo-nitrides (MCN), oxy-carbides (MCO), oxy-nitrides (MNO), oxy-carbo-nitrides (MCNO) of the metals (M) of the Group IVa of the Periodic Table (hereinafter referred to as M(CNO)). No more than 10% by weight of one or more metal oxide powders, 10 to 90% by weight of one or more M(CNO) powders, and balance Al₂ O₃, are blended and compacted into a green compact under conventional conditions. Thereafter, the compact is subjected to hot pressing at a temperature of 1580° C., a pressure of 30 atm. and a holding time of 10 to 30 minutes. The resulting ceramic has a composition substantially equal to the blending composition of the powders, and the metal oxide forms the binder phase thereof while Al₂ O₃ and M(CNO) define the hard dispersed phase.

The aforesaid Al₂ O₃ ceramic is, however, insufficient for use in cutting tools. More specifically, as the demands for labor saving, higher performance and higher speed operation increase in various industrial fields, the conditions under which the cutting tools are put to use are also becoming severe. Thus, cutting tools are now required to have greater strength and toughness. However, the aforesaid Al₂ O₃ ceramic, as well as the other kinds of ceramics, are susceptible to fracture or chipping due to insufficient strength and toughness when used under severe conditions.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an Al₂ O₃ ceramic which exhibits excellent strength and toughness, so that it is specifically suitable for use in cutting tools for use under severe cutting conditions.

According to the present invention, there is provided an Al₂ O₃ based ceramic essentially consisting of:

5% to 50% by weight of a hard dispersed phase composed of at least one compound which is selected from the group consisting of metal carbide, metal nitride, metal carbo-nitride, metal oxy-carbide, metal oxy-nitride and metal carbo-oxy-nitride, and is represented by M(CNO), wherein M is at least one metal selected from the group consisting of Ti, Zr and Hf;

optionally 1% to 25% by weight of ZrO₂ ; and

balance Al₂ O₃ matrix containing an additive dissolved in the Al₂ O₃ grains so as to form solid solution therewith, the additive being contained in an amount of 0.01% to 12% by weight with respect to the Al₂ O₃ matrix and being composed of at least one oxide of an element selected from the group consisting of Y, Mg, Cr, Ni, Co, and rare earth elements;

the Al₂ O₃ grains having an average grain size of no larger than 1.0 μm while the hard phase constituent has an average grain size of no larger than 0.6 μm.

DETAILED DESCRIPTION OF THE INVENTION

After an extensive study over the improvement of the conventional Al₂ O₃ based ceramic, the inventors have successfully obtained a novel Al₂ O₃ based ceramic which has excellent strength and toughness. More specifically, the inventors prepared Al₂ O₃ powder of an average particle size of no greater than 0.6 μm, metal oxide powders (additives) of an average particle size of no greater than 1 μm, and powders of hard dispersed phase constituents having an average particle size of no greater than 0.5 μm. The hard phase constituents were one or more compounds selected from the group consisting of metal carbide, metal nitride, metal carbo-nitride, metal oxy-carbide, metal oxy-nitride, metal carbo-oxy-nitride and their solid solutions, and represented by M(CNO), wherein M is a metal selected from the metals of the Group IVa of the Periodic Table, i.e., Ti, Zr and Hf. The metal oxides were those of a metal selected from the group consisting of Y, Mg, Cr, Ni, Co and rare earth elements (R). These starting powders were blended and mixed into the following composition:

Metal oxide powder: 0.01% to 10% by weight with respect to Al₂ O₃ powder

M(CNO) powder: 5% to 50% by weight with respect to the total amount

Al₂ O₃ powder: remainder

Then, the mixture was compacted into a green compact under the conventional conditions, and was sintered by holding it at a temperature of 1,680° C. to 1,830° C. in an argon atmosphere or an atmosphere of a mixture of argon and inert gasses at 0.8 to 5 atm for 10 to 40 minutes. With this high-temperature, brief sintering, it was confirmed that in the resulting Al₂ O₃ ceramic, the metal oxide was substantially dissolved in the Al₂ O₃ matrix to form solid solution therewith, and M(CNO) was dispersed in the matrix.

The Al₂ O₃ ceramic thus obtained was further heat-treated at a temperature of 1,300° C. to 1,450° C. for 2 to 10 hours by holding it in an argon atmosphere or an atmosphere of a mixture of argon and inert gasses at 100 to 2,000 atm. This heat treatment may be carried out following the sintering operation, or separately from the sintering. With this heat treatment, there was obtained a sintered ceramic with a very fine structure where the average grain size of Al₂ O₃ was no greater than 1 μm while that of M(CNO) was no greater than 0.6 μm, and the resulting ceramic exhibited great strength and toughness as compared with those of the prior art.

Thus, the Al₂ O₃ based ceramic in accordance with the present invention is characterized in that it essentially consists of 5% to 50% by weight of a hard phase constituent of at least one compound represented by M(CNO), wherein M is at least one metal of Ti, Zr and Hf; and balance Al₂ O₃ matrix containing an additive dissolved in the Al₂ O₃ grains so as to form solid solution therewith, the additive being contained in an amount of 0.01% to 10% by weight with respect to the Al₂ O₃ matrix and being composed of at least one oxide of Y, Mg, Cr, Ni, Co or rare earth elements. In addition, the Al₂ O₃ grains have an average grain size of no greater than 1.0 μm while the hard phase constituent has an average grain size of no greater than 0.6 μm.

The Al₂ O₃ ceramic in accordance with the invention has a very fine structure where the Al₂ O₃ grains and M(CNO) grains are both very fine. In addition, the metal oxides are dissolved in the Al₂ O₃ grains so as to form solid solution therewith. Therefore, the ceramic of the invention exhibits superior strength and toughness, so that it can be suitably used in cutting tools which are used under severe circumstances.

In addition, when the M(CNO) of the hard dispersed phase is especially metal carbo-nitride or metal carbo-oxy-nitride represented by M(C_(x) N_(y) O_(z)), wherein 0.95≦x+y+z≦1, 5/95≦x/y≦70/30 and 0≦z≦0.1, the sintered material possesses particularly fine structure and exhibits further enhanced strength and toughness.

With respect to the atomic ratios x, y and z in the M(C_(x) N_(y) O_(z)), if the ratios fall out of the range as specified above, the desired characteristics cannot be obtained. More specifically, if a hard phase constituent powder having the ratio x/y exceeding 70/30 is used, sintering operation becomes difficult, so that the sintering must be carried out at a higher temperature in order to ensure a sufficient degree of sintering. However, when the sintering temperature is elevated, the Al₂ O₃ grains grow excessively to the size of about 3 to 5 μm. On the other hand, if the powder to be used has the ratio x/y of less than 5/95, M(C_(x) N_(y) O_(z)) itself is subjected to an excessive grain growth, so that the hardness of the hard dispersed phase is unduly reduced, resulting in the ceramic of low strength. In addition, the atomic ratio z of oxygen in the M(C_(x) N_(y) O_(z)) is determined so as to satisfy the relationship of 0≦z≦0.1. However, if the ratio exceeds 0.1, the grain growth of M(C_(x) N_(y) O_(z)) becomes excessive although the higher degree of sintering is attained. Therefore, the atomic ratio z is preferably determined so as to be no greater than 0.1.

Furthermore, in the Al₂ O₃ ceramic of the invention, the M(CNO) grains may exist not only at the grain boundaries of Al₂ O₃ grains but also within Al₂ O₃ grains, and the M(CNO) grains existing in the Al₂ O₃ grains substantially enhance strength and toughness of the ceramic. In this regard, it is preferable that the M(CNO) grains exist within the Al₂ O₃ grains at least in an amount of 10% by weight with respect to the hard dispersed phase, and those within the Al₂ O₃ grains has an average grain size of no greater than 0.3 μm.

In addition, in the above Al₂ O₃ ceramic, due to the above specific structure, a great residual compressive stress is exerted on the Al₂ O₃ grains. Thus, it is preferable that a residual compressive stress of no less than 20 MPa be exerted on the Al₂ O₃ grains.

The reasons for the numerical restrictions in the resulting ceramic will now be described in conjunction with the procedures.

(a) Al₂ O₃

If the average grain size of Al₂ O₃ exceeds 1 μm, the Al₂ O₃ grains abruptly grow in such a manner as to take in the M(CNO), so that their average grain size becomes unduly large, resulting in a lowering of the strength of the ceramic. Therefore, the average grain size of Al₂ O₃ is determined so as to be no greater than 1 μm.

In this regard, in order to ensure the formation of Al₂ O₃ grains of the above grain size, the average particle size of the Al₂ O₃ powder should be no greater than 0.6 μm.

Furthermore, although not restricted in the above, it is preferable that the content of Al₂ O₃ in the ceramic be set to no less than 50% by weight in order to obtain a ceramic having sufficient heat resistance.

(b) M(CNO)

M(CNO) is caused to be dispersed at the grain boundaries of Al₂ O₃ to enhance the wear resistance of ceramic, and is partly caused to be dissolved in the Al₂ O₃ grains to improve the strength and toughness of the ceramic.

If the M(CNO) content is less than 5% by weight, the above advantages cannot be obtained. On the other hand, if its content in the ceramic exceeds 50% by weight, the degree of sintering is reduced, so that the strength of the ceramic is unduly lowered. Therefore, the M(CNO) content in the ceramic is determined so as to range from 5% to 50% by weight. In order to ensure this range, the blending composition of the M(CNO) powder is also set to the range of between 5% and 50% by weight.

Furthermore, if the average grain size of M(CNO) exceeds 0.6 μm, the interface at which the adjacent M(CNO) grains contact each other is increased. As a result, the resulting hard dispersed phase comes to have grains of greater sizes than expected, so that the strength and toughness of the ceramic is reduced. Accordingly, the average grain size of the hard phase constituent is determined to be no greater than 0.6 μm. In this regard, in order to ensure the above grain size of M(CNO) grains, it is preferable that the average particle size of the M(CNO) powder be no greater than 1.0 μm. Furthermore, in the case where the M(CNO) of the hard dispersed phase is carbo-nitride or carbo-oxy-nitride represented by M(C_(x) N_(y) O_(z)), wherein 0.95≦x+y+z≦1, 5/95≦x/y≦70/30 and 0≦z≦0.1, degree of sintering is further improved, and grain growth during the sintering is less, so that the above fine structure may be obtained more easily.

Moreover, as described above, it is preferable that the M(CNO) grains exist within the Al₂ O₃ grains at least in an amount of 10% by weight with respect to the hard dispersed phase, and those within the Al₂ O₃ grains have an average grain size of no greater than 0.3 μm. In order to produce the ceramic of this structure, the M(CNO) powder including at least 10% of fine powder of an particle size no greater than 0.3 μm should be used. This fine powder may be prepared by introducing M(CNO) powder of an average particle size of 0.7 to 2 μm in an attrition mill together with WC-based cemented carbide balls of 1 to 3 mm in diameter, and carrying out crushing for 10 to 100 hours.

(c) Metal oxides

The metal oxides are dissolved in the Al₂ O₃ to form solid solution therewith during the sintering operation to enhance the degree of sintering, so that the strength of the resulting ceramic is increased. In addition, the metal oxides facilitate the Al₂ O₃ grains to grow so as to take in fine M(CNO) grains of no greater than 0.3 μm. However, if the content is set to less than 0.01% by weight with respect to Al₂ O₃, the above effects cannot be obtained. On the other hand, if the content exceeds 10% by weight, it remains at the grain boundaries of the Al₂ O₃ grains, so that the hardness and strength at high temperature are particularly reduced. In this regard, the blending composition of the metal oxide powders should be also determined equal to the above range.

Furthermore, in order to cause the grains to be dissolved in the Al₂ O₃ more easily, it is preferable that the particle size of the metal oxide powder to be blended be no greater than 1 μm.

(d) Internal stress.

In some examples of the ceramic of the invention, it has been confirmed that the residual stress exerted on the Al₂ O₃ grains is comparatively great. This is due to the difference in expansion coefficients between the M(CNO) and the Al₂ O₃ matrix. For example, if only TiC is added, which has an expansion coefficient less than the Al₂ O₃, a comparatively small stress is exerted on the Al₂ O₃ in the resulting sintered material. However, when M(C_(x) N_(y) O_(z)), wherein 0.95≦x+y+z≦1, 5/95≦x/y≦70/30 and 0≦z≦0.1, is added, the residual compressive stress exerted on the Al₂ O₃ grains becomes great. In view of the properties of the ceramic, it is preferable that a residual compressive stress exerted on the Al₂ O₃ grains exceeds 20 MPa.

Next, the restrictions on the conditions for the manufacturing process will be explained.

In the procedures, the sintering is carried out by holding the mixture in an argon atmosphere or an atmosphere of a mixture of argon and inert gasses at 0.8 to 5 atm at a temperature of 1,680° C. to 1,830° C. for 10 to 40 minutes. With this high-temperature, brief sintering, the metal oxides are dissolved in the Al₂ O₃ matrix to form solid solution therewith. However, if the pressure, temperature and holding time are less than 0.8 atm, 1,680° C., and 10 minutes, respectively, sufficient sintering cannot be carried out, so that the metal oxides do not sufficiently dissolve in the Al₂ O₃ matrix. On the other hand, if the pressure exceeds 5 atm, gas generated during the sintering operation remains in the ceramic, so that fine sintered material cannot be obtained. In addition, if the temperature exceeds 1,830° C., the material powders are subjected to decomposition, so that the sintering is further adversely affected due to the gas produced by this decomposition. Furthermore, if the holding time exceeds 40 minutes, minute grains of no more than 0.3 μm among the M(CNO) grains are subjected to grain growth, so that the movement of the M(CNO) grains into the Al₂ O₃ matrix at the subsequent step is insufficient.

Furthermore, in the procedures, the sintered Al₂ O₃ ceramic is further heat-treated at a temperature of 1,300° C. to 1,450° C. for 2 to 10 hours by holding it in an argon atmosphere or an atmosphere of a mixture of argon and inert gasses at 100 to 2,000 atm. With this heat treatment, the pores in the structure are reduced to obtain a ceramic of generally true density, and the grain growth of the Al₂ O₃ grains is promoted. In addition, among the M(CNO) existing at the grain boundaries of Al₂ O₃ grains, those which have an average grain size of no greater than 0.3 μm are taken in the Al₂ O₃ matrix. Therefore, the strength and toughness of the resulting ceramic are improved. However, if the pressure in the atmosphere is less than 100 atm, the removal of the pores is insufficient. If the temperature is less than 1,300° C., the grain growth of the Al₂ O₃ is extremely slow. In addition, if the holding time is shorter than 2 hours, the amount of the M(CNO) taken in the Al₂ O₃ grains is insufficient. On the other hand, even if the pressure is set to above 2,000 atm, no further advantages can be obtained. If the temperature exceeds 1,450° C., the grain growth of M(CNO) occurs as the Al₂ O₃ grains grow, so that the content of M(CNO) dispersed in the Al₂ O₃ becomes unduly reduced. Furthermore, even if the holding time is increased above 10 hours, it results only in increased manufacturing costs.

In the Al₂ O₃ based ceramic as described above, the metal oxides are dissolved in the Al₂ O₃. However, some of them may remain at the grain boundaries of Al₂ O₃ in the form of compounds with Al₂ O₃. This compounds serve to further enhance the strength and toughness of the ceramic.

More specifically, Al₂ O₃ powder, metal oxide powders, and powders of hard dispersed phase constituents were blended and mixed in the following composition:

Metal oxide powder: 1% to 12% by weight with respect to Al₂ O₃ power

M(CNO) powder: 5% to 50% by weight with respect to the total

Al₂ O₃ powder: remainder

Then, the mixture was compacted and sintered in the same conditions as previously described, and further subjected to the heat treatment under the same conditions. With these procedures, the ceramic in which the compounds of Al₂ O₃ and the metal oxides (excluding Cr₂ O₃) are formed and remain at the grain boundaries of Al₂ O₃ grains is formed. The M(CNO) is dispersed both at the grain boundaries of Al₂ O₃ grains and within the grains at a specific ratio as described previously.

In the modified ceramic as described above, it is preferable that the content of the compounds of Al₂ O₃ and metal oxides range from 0.5% to 10% by weight. If the content is less than 0.5% by weight, the aforesaid effects cannot be obtained. On the other hand, if the content exceeds 10% by weight, the hardness and wear resistance are reduced.

Furthermore, the Al₂ O₃ based ceramic of the invention may be further modified using zirconium oxide (ZrO₂). More particularly, ZrO₂ powder having an average particle size of 0.5 μm is prepared, and blended with the other powders in the following composition:

Metal oxide powder: 0.01% to 10% by weight with respect to Al₂ O₃ power

ZrO₂ : 1% to 25% by weight with respect to the total amount

M(CNO) powder: 5% to 45% by weight with respect to the total amount

Al₂ O₃ powder: remainder

Then, the mixture was compacted into a green compact under the conventional conditions, and was sintered by holding it in a non-oxidizing atmosphere at a temperature of 1,600° C. to 1,800° C. for a prescribed period of time, and the sintered material was further heat-treated at a temperature of 1,400° C. to 1,800° C. for a prescribed period of time by holding it in an inert atmosphere at 500 to 2000 atm.

The Al₂ O₃ based ceramic thus produced essentially consists of a matrix of Al₂ O₃ in which one or more of the metal oxides are dissolved so as to form solid solution therewith; M(CNO) in an amount of 5% to 45% by weight (with respect to the Al₂ O₃ matrix), and ZrO₂ in an amount of 1% to 25% by weight in which one or more of the metal oxides are dissolved so as to form solid solution therewith. In addition, the ZrO₂ dissolved in the Al₂ O₃ grains has an average grain size of no greater than 0.2 μm, while the M(CNO) has an average grain size of no greater than 0.3 μm.

In the foregoing, ZrO₂ partly exists at the grain boundaries of Al₂ O₃ grains, and these grains are subjected to phase transformation from a tetragonal system to a monoclinic system to thereby impart residual compressive stress to the surface of the ceramic. Therefore, the ceramic is less susceptible to fracture or chipping when used as cutting tools. In addition, ZrO₂ is partly dissolved in the Al₂ O₃ grains during the heat treatment to thereby impart a desired residual stress to the Al₂ O₃ grains. However, if ZrO₂ content in the ceramic is less than 1% by weight, desired effect cannot be obtained. On the other hand, if the content exceeds 25% by weight, the resistance to thermal shock is deteriorated. Therefore, the ZrO₂ content in the ceramic is determined so as to range from 1% to 25% by weight. In this respect, the blending composition for the ZrO₂ powder should also be determined so as to range from 1% to 25% by weight.

Moreover, as a further modified ceramic in accordance with the present invention, there is provided a ceramic with a hard coating. The hard coating may be effected by introducing the ceramic substrate obtained as described above into a physical vapor deposition apparatus of the activation reaction type using an electron beam, as disclosed in Japanese patent Application laid open with Publication No. 63-26356. The coating operation is carried out by controlling the deposition rate so as to be slow. The rate of the conventional depositing process is usually 2 μm (in thickness) per hour, but in the invention, the vapor deposition is carried out, for example, at a rate of 0.1 to 0.6 μm per hour. One or more layers of at least one composition, selected from the group consisting of carbide, nitride and carbo-nitride of titanium and Al₂ O₃, are formed on the substrate under these conditions. The hard coating thus formed is very fine, and when the average grain size is reduced to no larger than 300 Angstroms, the bonding strength of the hard coating with respect to the substrate is greatly increased. Therefore, the resulting ceramic is less susceptible to fracture or chipping even when used under severe conditions, and exhibits superior antifracture characteristics over a long period of time.

Furthermore, the most preferable ceramic substrate to be coated is the one that has the following composition:

TiCN: 20% to 50% by weight

Metal oxide: 0.01% to 10% by weight with respect to Al₂ O₃

Al₂ O₃ : remainder

The average thickness of the hard coating should range from 0.3 to 20 μm in order to ensure adequate properties, while the average grain size of the coating should range from 150 to 300 Angstroms.

In the foregoing, TiCN serves to enhance the thermal shock resistance and to enhance the bonding strength between the substrate and the hard coating. However, if TiCN content is less than 20% by weight, the above advantages cannot be obtained. Therefore, the TiCN content is determined so as to range from 20 to 50% by weight. In addition, it is further preferable that TiCN have atomic ratios satisfying 5/95≦C/N≦70/30.

In addition, the average grain size of the coating should range from 150 to 300 Angstroms. If it exceeds 300 Angstroms, antifracture characteristics are sharply reduced, so that the ceramic is susceptible to fracture or chipping under severe cutting conditions. Therefore, the finer the grains, the better the antifracture characteristics. However, if the average grain size is less than 150 Angstroms, the rate of formation of the coating layers is sharply reduced, resulting in high in cost. Therefore, the average grain size has been determined as specified above.

Furthermore, the average thickness of the hard coating should range from 0.3 to 20 μm. If the thickness is less than 0.3 μm, sufficient wear resistance cannot be obtained. On the other hand, if the thickness exceeds 20 μm, the thermal shock resistance is reduced unduly.

The vapor deposition process to be used may be either physical vapor deposition or chemical vapor deposition, although the deposition rate has to be controlled as described above.

As described above, the Al₂ O₃ based ceramic in accordance with the present invention has great strength and toughness. Accordingly, when employed in cutting tools for use under conditions where the above characteristics are required, the ceramics of the invention exhibit excellent performance for an extremely long period of time. Therefore, although high-speed operation demands are increasing in the field of cutting tools, the cutting tools of the ceramics of the invention can successfully meet these demands.

The present invention will now be described in more detail by way of the following examples.

EXAMPLE 1

There were prepared starting powders of Al₂ O₃, M(CNO) and metal oxides, each having a prescribed average particle size as shown in Tables 1-1a and 1-2a. These powders were blended in various blending compositions as set forth in Tables 1-1a and 1-2a, and were subjected to wet mixing in a ball mill for a period of 72 hours. After being dried, the mixtures were pressed under a pressure of 1 ton/cm² into green compacts having a size of 30 mm×30 mm×10 mm, and into green compacts having a shape of a cutting insert in conformity with SNGN120408 of the ISO standards. Subsequently, these green compacts were subjected to high-temperature, short-time sintering and the heat treatments under the conditions as set forth in Tables 1-1b and 1-2b, to produce ceramics 1 to 15 of the invention.

Furthermore, for comparison purposes, green compacts having the compositions as set forth in Table 1-2a were prepared using powders shown in Table 1-2a, and were subjected to hot pressing under the conditions of Table 1-2b instead of being subjected to the high-temperature short-time sintering and heat treatments. Thus, comparative ceramics 1 to 5 were manufactured.

As to the various ceramics thus obtained, their compositions, theoretical density ratios, Rockwell hardnesses (A scale) and transverse rupture strengths were measured. In addition, fracture toughnesses were also measured by the indentation method.

Subsequently, the ceramics thus obtained were subjected to wet-type interrupted cutting tests of hardened steel under the following conditions:

Workpiece: hollow square bar of hardened steel (JIS.SUJ2; Hardness: H_(R) C61)

Cutting speed: 200 meters/minute

Depth of cut: 0.3 mm

Feed rate: 0.2 mm/revolution

In this test, the time required until the cutting edge was subjected to chipping was measured. The results are set forth in Table 2.

As will be clearly seen from Table 2, the ceramics 1 to 15 of the invention had theoretical density ratios of no less than 99% and had less micropore formation, so that the ceramics are very fine. Therefore, as compared with the comparative ceramics 1 to 5, the ceramics 1 to 15 of the invention exhibited excellent strength and toughness, and high strength as well.

EXAMPLE 2

There were prepared starting powders of Al₂ O₃, M(CNO) and metal oxides, each having a prescribed average particle size as shown in Tables 3-1a and 3-2a. These powders were blended in various blending compositions as set forth in Tables 3-1a and 3-2a, and were subjected to wet mixing in a ball mill for a period of 72 hours. After being dried, the mixtures were pressed under a pressure of 1 ton/cm² into green compacts having a size of 30 mm×30 mm×10 mm, and green compacts having a diameter of 120 mm and a width of 60 mm. Subsequently, these green compacts were subjected to high-temperature and short-time sintering and the heat treatments under the conditions as set forth in Tables 3-1b and 3-2b, to produce ceramics 16 to 30 of the invention.

Furthermore, for comparison purposes, green compacts having the compositions as set forth in Table 3-2a were prepared using powders shown in Table 3-2a, and were subjected to hot pressing under the conditions of Table 3-2b instead of being subjected to the high-temperature and short-time sintering and heat treatments. Thus, comparative ceramics 6 to 10 were manufactured.

As to the various ceramics thus obtained, their compositions, theoretical density ratios, Rockwell hardnesses (A scale) and transverse rupture strengths were measured. In addition, fracture toughnesses were also measured by the indentation method.

Furthermore, an L-shaped bore having a length of 30 mm was formed through the disk-shaped ceramics 120 mm in diameter by using an electric discharge machine. Then, using the resulting ceramics as dies, warm extrusion operation of aluminum was carried out under the following conditions:

Workpiece: pure aluminum (heating temperature: 480° C.)

Extrusion speed: 20 meters/minute

Measurements were taken of the extrusion time elapsed before the dies cracked and service life was terminated. The results are set forth in Table 4.

As will be clearly seen from the results of Table 4, the ceramics 16 to 30 of the invention had theoretical density ratios of no less than 99% and had less micropore formation, so that they were very fine. As compared with the comparative ceramics 16 to 30, the ceramics 16 to 30 of the invention exhibited excellent strength and toughness, and high strength as well.

EXAMPLE 3

There were prepared starting powders of Al₂ O₃, M(CNO), ZrO₂ and other metal oxides, each having a prescribed average particle size as shown in Table 5-1. These powders were blended in various blending compositions as set forth in Table 5-1, and were subjected to wet mixing in an attrition mill for a period of 30 hours. After being dried, the mixtures were pressed under a pressure of 1 ton/cm² into green compacts each having a size of 30 mm×30 mm×10 mm, and green compacts each having a shape of a cutting insert, in conformity with SNGN120408 of the ISO standards. Subsequently, these green compacts were subjected to sintering and heat treatments under the conditions as set forth in Table 5-2, to produce ceramics 31 to 40 of the invention.

Furthermore, for comparison purposes, green compacts having the compositions as set forth in Table 5-1 were prepared using powders shown in Table 5-1, and were subjected to hot pressing under the conditions of Table 5-2 instead of being subjected to the sintering and heat treatments. Thus, comparative ceramics of 11 to 15 were manufactured.

As to the various ceramics thus obtained, their compositions, theoretical density ratios, and Rockwell hardnesses (A scale) were measured. In addition, in order to evaluate strength and toughness, bending strength at three-points and fracture toughness using the indentation method were also measured.

Subsequently, the ceramics thus obtained were subjected to dry-type continuous cutting tests of high speed steel under the following conditions:

Workpiece: high speed steel (SKD-11)

Cutting speed: 100 meters/minute

Depth of Cut: 1 mm

Feed rate: 0.13 mm/revolution

In this test, the length of time it took until the flank wear of the cutting edge reached 0.2 mm wide was measured. The results are set forth in Table 6.

As will be clearly seen from Table 6, the ceramics 31 to 40 of the invention had theoretical density ratios of no less than 99% and had less micropore formation, so that the ceramics of the invention are very fine. In addition, the ceramics have great strength due to the residual stress of the Al₂ O₃ grains. Furthermore, ZrO₂ existing at the grain boundaries of the Al₂ O₃ grains enhances the toughness, so that the cutting edge is less susceptible to fracture or chipping, resulting in an increased tool life. Therefore, as compared with the comparative ceramics 11 to 13, the ceramics 31 to 40 of the invention exhibit excellent strength and toughness, and high strength as well.

EXAMPLE 4

There were prepared starting powders of TiCN (average particle size: 0.5 μm), Al₂ O₃ (0.3 μm), MgO (0.3 μm), Y₂ O₃ (0.6 μm) and ZrO₂ (0.3 μm). These powders were blended in various blending compositions as set forth in Tables 7-1a and 7-2a, and were subjected to wet mixing in an attrition mill for a period of 10 hours. After being dried, the mixtures were pressed into green compacts, which were then subjected to high-temperature and short-time sintering and the heat treatments under the conditions similar to those in the previous examples, to produce the ceramic substrates which were of a shape of a cutting insert in conformity with SNGN 120408.

Subsequently, these substrates were introduced into a physical vapor deposition apparatus of the activation reaction type using an electron beam, as disclosed in Japanese Patent Application laid open with Publication No. 63-26356. By controlling the deposition speed, the average grain size of the hard coating were varied, so that the hard coatings having compositions, average grain sizes, and average layer thicknesses, as set forth in Tables 7-1b and 7-2b, were formed.

Thereafter, the ceramics thus obtained (coated ceramics 1 to 10 of the invention and comparative coated ceramics 1 to 10) were subjected to wet-type interrupted cutting tests of hardened steel under the following conditions:

Workpiece: round bar of SNCM 439

Cutting speed: 350 meters/minutes

Feed rate: 0.3 mm/revolution

Depth of cut: 1.5 mm

Cutting time: 20 minutes

Furthermore, the high-speed dry-type continuous cutting test was carried out under the following conditions:

Workpiece: round bar of SNCM 439 having four axial grooves disposed in longitudinally equally spaced relation to one another

Cutting speed: 250 meters/minute

Feed rate: 0.1 mm/revolution

Depth of cut: 1.0 mm

Cutting time: 15 minutes

In these tests, the flank wear and face wear of the cutting edge were measured, and the results are set forth in Table 8.

As will be clearly seen from Table 8, in the coated ceramics 1 to 10 of the invention, the average grain size of the hard coating ranged from 150 to 300 Angstroms. Since the hard coating is thus minute, the ceramics exhibited superior antifracture characteristics even in the severe cutting conditions such as high speed cutting or high feed cutting, and hence exhibits a prolonged tool life. In contrast, the comparative coated ceramics 1 to 10 were subjected to chipping during the cutting operation.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

                                      TABLE 1-1a                                   __________________________________________________________________________            Blending composition (% by weight)                                             Metal oxides  M (CNO)                Al.sub.2 O.sub.3                          Average       Average                                                                             Percentage        Average                                   particle      particle                                                                            of powder         particle                                  size          size no larger         size                                      (μm)                                                                             Composition                                                                             (μm)                                                                             than 0.3 μm                                                                       Composition (μm)                            __________________________________________________________________________     Ceramics of                                                                    the invention                                                                  1       0.03                                                                               Y.sub.2 O.sub.3                                                                       0.3                                                                              0.8  15    TiC       30                                                                               0.3  remainder                     2      0.8  MgO    1.0                                                                              1.5  10    TiN       30                                                                               0.6  remainder                     3      0.5  Cr.sub.2 O.sub.3                                                                      2.0                                                                              0.5  25    TiC.sub.0.5 N.sub.0.5                                                                    30                                                                               0.3  remainder                     4      1.0  Nd.sub.2 O.sub.3                                                                      0.8                                                                              0.2  80    TiC.sub.0.43 N.sub.0.5 O.sub.0.07                                                        50                                                                                0.05                                                                               remainder                     5      0.8  CoO    0.3                                                                              0.5  25    (Ti,Zr)C  10                                                                               0.1  remainder                            0.8  Y.sub.2 O.sub.3                                                                       0.5                                                         6      1.0  CeO.sub.2                                                                             2.0                                                                              0.6  15    TiC       20                                                                               0.1  remainder                                          0.6  15    HfC.sub.0.5 N.sub.0.5                                                                    20                                   7      0.5  La.sub. 2 O.sub.3                                                                     0.1                                                                              0.1  100   TiC.sub.0.9 O.sub.0.1                                                                    15                                                                                0.05                                                                               remainder                     8      0.1  Y.sub.2 O.sub.3                                                                       0.5                                                                              0.3  40    (Ti,Hf)C.sub.0.5 N.sub.0.5                                                               20                                                                               0.3  remainder                            0.1  MgO    1.0                                                         9      0.2  MgO    0.5                                                                              0.5  30    (Zr,Hf)C.sub.0.4 N.sub.0.5 O.sub.0.1                                                     25                                                                               0.3  remainder                            0.2  CeO.sub.2                                                                             0.5                                                                              0.5  30    TiN.sub.0.9 O.sub.0.1                                                                     5                                   10      0.05                                                                               Dy.sub.2 O.sub.3                                                                      0.3                                                                              0.5  20    ZrC       20                                                                               0.1  remainder                     __________________________________________________________________________

                                      TABLE 1-1b                                   __________________________________________________________________________            Sintering               Heat treatment                                         Composition             Composition                                            of                 Holding                                                                             of                 Holding                             atmosphere                                                                            Pressure                                                                            Temperature                                                                           time atmosphere                                                                            Pressure                                                                            Temperature                                                                           time                                (%)    (Atm)                                                                               (°C.)                                                                          (min)                                                                               (%)    (Atm)                                                                               (°C.)                                                                          (Hr)                         __________________________________________________________________________     Ceramics of                                                                    the invention                                                                  1      Ar:100 1    1730   30   Ar:100 1500 1400   5                            2      Ar:100 1    1780   30   Ar:100 1500 1400   5                            3      Ar:90, 0.9  1800   30   Ar:100 1500 1350   5                                   N.sub.2 :10                                                             4      Ar:90, 0.9  1700   30   Ar:100 1500 1350   5                                   N.sub.2 :10                                                             5      Ar:100 1    1730   30   Ar:100 1500 1450   8                            6      Ar:100 1    1730   30   Ar:100 1500 1400   4                            7      Ar:100 0.95 1700   30   Ar:100 1500 1400   2                            8      Ar:90, 1    1780   30   Ar:90  1000 1400   4                                   He:10                   He:10                                           9      Ar:90  1    1780   30   Ar:90  1000 1450   4                                   He:10                   He:10                                           10     Ar:95, 2    1800   10   Ar:95  1000 1300   4                                   He:5                    He:5                                            __________________________________________________________________________

                                      TABLE 1-2a                                   __________________________________________________________________________            Blending composition (% by weight)                                             Metal oxides   M (CNO)                Al.sub.2 O.sub.3                         Average        Average                                                                             Percentage        Average                                  particle       particle                                                                            of powder         particle                                 size           size no larger         size                                     (μm)                                                                             Composition                                                                              (μm)                                                                             than 0.3 μm                                                                       Composition (μm)                           __________________________________________________________________________     Ceramics of                                                                    the invention                                                                  11     0.5  Yb.sub.2 O.sub.3                                                                      0.3                                                                               0.2  70    (Zr,Hf)C.sub.0.4 N.sub.0.5 O.sub.0.1                                                     10                                                                               0.3  remainder                    12     0.4  NiO    0.2                                                                               0.5  25    ZrC.sub.0.3 N.sub.0.7                                                                    40                                                                               0.3  remainder                           0.4  Yb.sub.2 O.sub.3                                                                      0.5                                                         13     0.5  Y.sub.2 O.sub.3                                                                       0.05                                                                              0.5   5    ZrC.sub.0.5 N.sub.0.45 O.sub.0.05                                                        10                                                                               0.3  remainder                           0.5  Dy.sub.2 O.sub.3                                                                      0.05                                                        14     0.05 Y.sub.2 O.sub.3                                                                       0.05                                                                              0.1  70    HfC.sub.0.7 N.sub.0.3                                                                     5                                                                               0.05 remainder                    15     0.1  Dy.sub.2 O.sub.3                                                                      0.5                                                                               0.3  35    TiC.sub.0.1 N.sub.0.9                                                                    35                                                                               0.3  remainder                    Prior art                                                                      cermets                                                                         1     0.03 Y.sub.2 O.sub.3                                                                       8  0.8   3    TiC       30                                                                               1.0  remainder                     2     0.8  MgO    3  1.0   1    TiN       10                                                                               1.0  remainder                     3     1.0  NiO    5  0.8   3    TiC.sub.0.5 N.sub.0.45 O.sub.0.05                                                        10                                                                               0.8  remainder                     4     0.5  Y.sub.2 O.sub.3                                                                       4  1.5   1    ZrC.sub.0.5 N.sub.0.5                                                                    30                                                                               1.5  remainder                           0.5  Yb.sub.2 O.sub.3                                                                      4                                                            5     0.1  Dy.sub.2 O.sub.3                                                                      5  2.0   1    HfC       30                                                                               1.5  remainder                    __________________________________________________________________________

                                      TABLE 1-2b                                   __________________________________________________________________________            Sintering or Hot pressing                                                                              Heat treatment                                         Composition             Composition                                            of                 Holding                                                                             of                 Holding                             atmosphere                                                                            Pressure                                                                            Temperature                                                                           time atmosphere                                                                            Pressure                                                                            Temperature                                                                           time                                (%)    (Atm)                                                                               (°C.)                                                                          (min)                                                                               (%)    (Atm)                                                                               (°C.)                                                                          (Hr)                         __________________________________________________________________________     Ceramics of                                                                    the invention                                                                  11     Ar:95  5    1700   20   Ar:95, 1000 1300   10                                  He:5                    He:5                                            12     Ar:100 0.95 1780   40   Ar:100 2000 1400   5                            13     Ar:100 0.8  1680   20   Ar:100 2000 1350   8                            14     Ar:100 0.9  1680   40   Ar:100 2000 1300   8                            15     Ar:100 1    1780   40   Ar:100 2000 1450   2                            Prior art                                                                      cermets                                                                         1     N.sub.2                                                                               100  1580   30   --     --   --     --                            2     Vacuum 0.5  1750   30   --     --   --     --                            3     N.sub.2                                                                               100  1600   30   --     --   --     --                            4     Ar     1    1600   30   --     --   --     --                            5     Ar     1    1600   30   --     --   --     --                           __________________________________________________________________________

                                      TABLE 2                                      __________________________________________________________________________     Composition (% by weight)                                                      M (CNO)               Al.sub.2 O.sub.3                                                                               Theo-                                    Average       At            Average   retical             Cutting              grain         grain         grain                                                                               Internal                                                                            density                                                                            Hard-                                                                              Bending                                                                              Fracture                                                                             time until           size      Within                                                                             bound-                                                                             Binder    size stress                                                                              ratio                                                                              ness                                                                               strength                                                                             toughness                                                                            chipping             (μm)   grains                                                                             ary phase                                                                              Al.sub.2 O.sub.3                                                                     (μm)                                                                             (MPa)                                                                               (%) (H.sub.R A)                                                                        (kg/mm.sup.2)                                                                        (MN/m.sup.3/2)                                                                       (min)                __________________________________________________________________________     Ceramics                                                                       of the                                                                         invention                                                                      1    0.6  3   27  --  remainder                                                                            0.8   10* 99.8                                                                               94.5                                                                               136   5.2   4.0                  2    0.6  2   28  --  remainder                                                                            0.9  13   99.5                                                                               94.0                                                                               122   5.0   3.0                  3    0.5  4   26  --  remainder                                                                            0.9  90   99.8                                                                               94.3                                                                               128   5.2   3.0                  4    0.4  20  30  --  remainder                                                                            0.6  60   99.6                                                                               94.6                                                                               122   6.0   4.3                  5    0.5  2    8  --  remainder                                                                            0.8  14   99.9                                                                               93.7                                                                               130   5.0   3.0                  6    0.6  4   36  --  remainder                                                                            0.9   8   99.8                                                                               94.4                                                                               122   5.0   3.0                  7    0.4  13   2  --  remainder                                                                            0.5   7   99.8                                                                               93.8                                                                               132   5.8   3.5                  8    0.4  6   14  --  remainder                                                                            0.6  120  99.9                                                                               94.0                                                                               138   5.4   4.0                  9    0.6  6   24  --  remainder                                                                            0.8  30   99.6                                                                               94.3                                                                               130   5.6   3.5                  10   0.6  3   17  --  remainder                                                                            0.9   20* 99.8                                                                               94.1                                                                               136   5.2   3.5                  11   0.4  6    4  --  remainder                                                                            0.7  110  99.8                                                                               93.8                                                                               136   5.6   3.0                  12   0.6  7   33  --  remainder                                                                            0.8  140  99.6                                                                               94.6                                                                               128   5.4   3.5                  13   0.6  0   10  --  remainder                                                                            0.8  55   99.8                                                                               93.9                                                                               136   5.2   3.0                  14   0.3  3    2  --  remainder                                                                            0.9  70   99.9                                                                               93.7                                                                               138   5.4   3.0                  15   0.5  8   27  --  remainder                                                                            1.0  80   99.8                                                                               94.5                                                                               132   5.4   3.5                  Prior                                                                          art                                                                            cermets                                                                        1    0.9  --  30  4   remainder                                                                            3.2   6   99.7                                                                               94.2                                                                                92   3.0   0.2                  2    1.6  --  10  2   remainder                                                                            2.8  18   99.2                                                                               92.9                                                                                78   3.4   0.1                  3    0.8  --  10  3   remainder                                                                            1.8  30   99.8                                                                               93.3                                                                                84   3.2   0.1                  4    1.5  --  30  4   remainder                                                                            3.5  25   99.8                                                                               93.5                                                                                82   3.0   0.2                  5    1.9  --  30  2   remainder                                                                            3.1   15* 99.7                                                                               93.9                                                                                78   3.0   0.2                  __________________________________________________________________________      *indicates tensile stress.                                               

                                      TABLE 3-1a                                   __________________________________________________________________________            Blending composition (% by weight)                                             Metal oxides  M (CNO)                Al.sub.2 O.sub.3                          Average       Average                                                                             Percentage        Average                                   particle      particle                                                                            of powder         particle                                  size          size no larger         size                                      (μm)                                                                             Composition                                                                             (μm)                                                                             than 0.3 μm                                                                       Composition (μm)                            __________________________________________________________________________     Ceramics of                                                                    the invention                                                                  16     0.03 Y.sub.2 O.sub.3                                                                       1 0.6  15    TiC       30                                                                               0.05 remainder                     17     0.5  Y.sub.2 O.sub.3                                                                       6 0.5  25    TiC.sub.0.5 N.sub.0.5                                                                    50                                                                               0.3  remainder                     18     0.2  MgO    3 0.2  80    TiC.sub.0.45 N.sub.0.5 O.sub.0.05                                                        30                                                                               0.1  remainder                     19     1.5  MgO    5 0.5  20    ZrC       20                                                                               0.6  remainder                     20     1.0  Nd.sub.2 O.sub.3                                                                      5 0.5  25    (Ti,Zr)C  10                                                                               0.6  remainder                     21     1.0  CeO.sub.2                                                                             3 0.1  100   TiC.sub.0.8 O.sub.0.2                                                                     5                                                                               0.6  remainder                     22     0.5  La.sub.2 O.sub.3                                                                      3 0.3  40    (Zr,Hf)C.sub.0.3 N.sub.0.7                                                               10                                                                               0.3  remainder                     23     0.05 Dy.sub.2 O.sub.3                                                                      3 0.5  20    (Zr,Hf)C  30                                                                               0.1  remainder                     24     0.5  Dy.sub.2 O.sub.3                                                                      5 0.5  30    (Ti,Zr)C.sub.0.7 N.sub.0.3                                                               30                                                                               0.3  remainder                     25     0.5  Yb.sub.2 O.sub.3                                                                      5 0.4  40    (Ti,Zr)C.sub.0.4 N.sub.0.5 O.sub.0.1                                                     30                                                                               0.3  remainder                     __________________________________________________________________________

                                      TABLE 3-1b                                   __________________________________________________________________________            Sintering               Heat treatment                                         Composition             Composition                                            of                 Holding                                                                             of                 Holding                             atmosphere                                                                            Pressure                                                                            Temperature                                                                           time atmosphere                                                                            Pressure                                                                            Temperature                                                                           time                                (%)    (Atm)                                                                               (°C.)                                                                          (min)                                                                               (%)    (Atm)                                                                               (°C.)                                                                          (Hr)                         __________________________________________________________________________     Ceramics of                                                                    the invention                                                                  16     Ar:100 1    1730   30   Ar:100 1500 1300   5                            17     Ar:100 1    1800   40   Ar:100 1500 1350   5                            18     Ar:100 1    1730   30   Ar:100 1500 1350   5                            19     Ar:100 0.9  1750   15   Ar:100 1500 1400   5                            20     Ar:100 0.9  1730   15   Ar:100 1000 1400   8                            21     Ar:100 0.9  1700   30   Ar:100 1000 1400   2                            22     Ar:90, 4    1750   10   Ar:90, 1500 1350   8                                   He:10                   He:10                                           23     Ar:90, 2    1730   30   Ar:90, 2000 1350   8                                   He:10                   He:10                                           24     Ar:100 1    1780   30   Ar:100 1000 1350   5                            25     Ar:100 0.9  1780   30   Ar:100 1000 1350   5                            __________________________________________________________________________

                                      TABLE 3-2a                                   __________________________________________________________________________            Blending composition (% by weight)                                             Metal oxides  M (CNO)                Al.sub.2 O.sub.3                          Average       Average                                                                             Percentage        Average                                   particle      particle                                                                            of powder         particle                                  size          size no larger         size                                      (μm)                                                                             Composition                                                                             (μm)                                                                             than 0.3 μm                                                                       Composition (μm)                            __________________________________________________________________________     Ceramics of                                                                    the invention                                                                  26     0.03 Y.sub.2 O.sub.3                                                                        3                                                                               0.4  30    ZrC.sub.0.5 N.sub.0.5                                                                    30                                                                               0.3  remainder                            1.0  NiO     2                                                          27     1.0  Nd.sub.2 O.sub.3                                                                       3                                                                               0.4  25    (Zr,Hf)C.sub.0.5 N.sub.0.5                                                               30                                                                               0.3  remainder                            1.0  CoO     2                                                          28     0.03 Y.sub.2 O.sub.3                                                                        3                                                                               0.2  80    TiC.sub.0.3 N.sub.0.7                                                                    30                                                                               0.3  remainder                            0.2  MgO     3                                                          29     0.2  MgO     3                                                                               0.5  20    ZrC       30                                                                               0.3  remainder                            1.0  CeO.sub.2                                                                              5                                                          30     0.05 Dy.sub.2 O.sub.3                                                                       3                                                                               0.1  100   TiC.sub.0.9 O.sub.0.1                                                                    30                                                                               0.3  remainder                            1.0  Cr.sub.2 O.sub.3                                                                       3                                                          Prior art                                                                      cermets                                                                         6     0.03 Y.sub.2 O.sub.3                                                                       15                                                                               0.8   2    TiC       30                                                                               1.0  remainder                      7     1.5  MgO    15                                                                               1.0   1    TiN       10                                                                               1.0  remainder                      8     0.5  La.sub.2 O.sub.3                                                                      15                                                                               0.8   2    TiC.sub.0.4 N.sub.0.4 O.sub.0.1                                                          10                                                                               0.8  remainder                      9     0.5  Y.sub.2 O.sub.3                                                                        9                                                                               1.5   1    ZrC.sub.0.5 N.sub.0.5                                                                    30                                                                               1.5  remainder                            1.5  NiO     6                                                          10     0.1  Dy.sub.2 O.sub.3                                                                       8                                                                               2.0   1    HfC       30                                                                               1.5  remainder                            1.5  Cr.sub.2 O.sub.3                                                                       7                                                          __________________________________________________________________________

                                      TABLE 3-2b                                   __________________________________________________________________________            Sintering or Hot pressing                                                                              Heat treatment                                         Composition             Composition                                            of                 Holding                                                                             of                 Holding                             atmosphere                                                                            Pressure                                                                            Temperature                                                                           time atmosphere                                                                            Pressure                                                                            Temperature                                                                           time                                (%)    (Atm)                                                                               (°C.)                                                                          (min)                                                                               (%)    (Atm)                                                                               (°C.)                                                                          (Hr)                         __________________________________________________________________________     Ceramics of                                                                    the invention                                                                  26     Ar:100 1    1730   20   Ar:100 1500 1350   8                            27     Ar:100 1    1780   30   Ar:100 1500 1350   8                            28     Ar:100 1    1730   20   Ar:100 1500 1300   5                            29     Ar:100 1    1780   30   Ar:100 1500 1350   8                            30     Ar:100 1    1750   20   Ar:100 1500 1300   8                            Prior art                                                                      cermets                                                                         6     N.sub.2                                                                               100 Torr                                                                            1580   30   --     --   --     --                            7     Vacuum  0.5 Torr                                                                           1750   30   --     --   --     --                            8     N.sub.2                                                                               100 Torr                                                                            1600   30   --     --   --     --                            9     Ar     1    1600   30   --     --   --     --                           10     Ar     1    1600   30   --     --   --     --                           __________________________________________________________________________

                                      TABLE 4                                      __________________________________________________________________________     Composition (% by weight)                                                      M (CNO)               Al.sub.2 O.sub.3                                                                               Theo-                                    Average       At            Average   retical             Cutting              grain         grain         grain                                                                               Internal                                                                            density                                                                            Hard-                                                                              Bending                                                                              Fracture                                                                             time until           size      Within                                                                             bound-                                                                             Com-      size stress                                                                              ratio                                                                              ness                                                                               strength                                                                             toughness                                                                            chipping             (μm)   grains                                                                             ary pounds                                                                             Al.sub.2 O.sub.3                                                                     (μm)                                                                             (MPa)                                                                               (%) (H.sub.R A)                                                                        (kg/mm.sup.2)                                                                        (MN/m.sup.3/2)                                                                       (min)                __________________________________________________________________________     Ceramics                                                                       of the                                                                         invention                                                                      16   0.6  26.0                                                                               4.0   0.5                                                                              remainder                                                                            0.8   3   99.8                                                                               93.9                                                                               145   6.2   16                   17   0.6  39.8                                                                               10.2                                                                               10  remainder                                                                            0.9  96   99.9                                                                               93.6                                                                               142   6.0   8                    18   0.4  8.5 21.5                                                                                 3.5                                                                              remainder                                                                            0.7  54   99.7                                                                               93.7                                                                               146   6.4   16                   19   0.6  17.0                                                                               3.0 8   remainder                                                                            0.9   1   99.6                                                                               93.4                                                                               144   6.4   16                   20   0.6  9.8 0.2 5   remainder                                                                            0.8    13*                                                                               99.8                                                                               93.7                                                                               140   6.2   8                    21   0.4  2.0 3.0   0.5                                                                              remainder                                                                            0.9   4   99.8                                                                               93.9                                                                               142   6.2   12                   22   0.4  6.5 3.5 2   remainder                                                                            0.6  130  99.9                                                                               93.6                                                                               142   6.0   8                    23   0.6  25.5                                                                               4.5   1.5                                                                              remainder                                                                            0.6  17   99.9                                                                               94.0                                                                               140   6.2   12                   24   0.6  22.6                                                                               7.6 4   remainder                                                                            0.7  108  99.8                                                                               93.6                                                                               143   6.0   12                   25   0.6  20.2                                                                               9.8 5   remainder                                                                            0.8  75   99.8                                                                               93.5                                                                               144   6.2   12                   26   0.5  23.6                                                                               6.4 4   remainder                                                                            0.7  47   99.7                                                                               93.7                                                                               142   6.2   8                    27   0.5  24.3                                                                               5.7 6   remainder                                                                            0.7  67   99.7                                                                               93.4                                                                               142   6.0   8                    28   0.3  10.5                                                                               19.5                                                                               5   remainder                                                                            0.5  170  99.7                                                                               93.6                                                                               146   6.4   16                   29   0.6  26.4                                                                               3.6 7   remainder                                                                            0.8   8   99.8                                                                               93.4                                                                               140   5.4   8                    30   0.4  5.5 24.5                                                                               1   remainder                                                                            0.9  25   99.9                                                                               93.8                                                                               146   6.2   16                   Prior                                                                          art                                                                            cermets                                                                         6   1.1  --  30   6**                                                                               remainder                                                                            2.1   21* 99.8                                                                               93.2                                                                                82   3.2   2                     7   1.6  --  10   8**                                                                               remainder                                                                            2.5   7   99.7                                                                               93.1                                                                                78   3.0   2                     8   1.3  --  10   6**                                                                               remainder                                                                            1.7  40   99.7                                                                               93.2                                                                                84   3.4   4                     9   1.8  --  30   5**                                                                               remainder                                                                            2.7  35   99.8                                                                               93.2                                                                                70   3.0   2                    10   2.5  --  30   8**                                                                               remainder                                                                            2.4   16* 99.8                                                                               93.4                                                                                72   3.0   2                    __________________________________________________________________________      *indicates tensile stress.                                                     **indicates binder phase.                                                

                                      TABLE 5-1                                    __________________________________________________________________________     Blending composition (% by weight)                                             M (CNO)                   ZrO.sub.2    Metal oxides Al.sub.2 O.sub.3           Average   Percentage      Average                                                                             Percentage                                                                             Average      Average                    particle  of fine         particle                                                                            of fine particle     particle                   size      powder          size powder  size Com-    size                       (μm)   (wt %)                                                                               Composition                                                                              (μm)                                                                             (wt %)  (μm)                                                                             position                                                                               (μm)                    __________________________________________________________________________     Ceramics                                                                       of the                                                                         invention                                                                      31   0.5  15    TiC.sub.0.7 N.sub.0.3                                                                   5                                                                               0.4  30    10                                                                               0.5  Y.sub.2 O.sub.3                                                                      0.4                                                                              0.6  remainder             32   0.6  12    TiC.sub.0.5 N.sub.0.5                                                                  30                                                                               0.3  40    10                                                                               0.6  Dy.sub.2 O.sub.3                                                                     0.4                                                                              0.4  remainder             33   0.4  20    TiC.sub.0.3 N.sub.0.7                                                                  45                                                                               0.5  20     1                                                                               0.6  Sm.sub.2 O.sub.3                                                                     0.4                                                                              0.6  remainder             34   0.5  15    ZrC.sub.0.5 N.sub.0.5                                                                  20                                                                               0.3  30    25                                                                               0.4  Yb.sub.2 O.sub.3                                                                     0.5                                                                              0.3  remainder             35   0.3  20    TiN     30                                                                               0.4  30    10                                                                               0.4  Y.sub.2 O.sub.3                                                                      0.3                                                                              0.4  remainder             36   0.5  20    TiC.sub.0.5 N.sub.0.4 O.sub.0.1                                                        30                                                                               0.4  30    10                                                                               0.4  Y.sub.2 O.sub.3                                                                      0.3                                                                              0.3  remainder             37   0.3  20    TiC     30                                                                               0.4  30    10                                                                               0.1  MgO   0.5                                                                              0.4  remainder             38   0.5  20    ZrC.sub.0.7 N.sub.0.2 O.sub.0.1                                                        25                                                                               0.4  30    10                                                                               0.1  CaO   0.5                                                                              0.3  remainder                  0.5  20    HfC.sub.0.7 N.sub.0.2 O.sub.0.1                                                         5                                                     39   0.5  20    TiC.sub.0.7 N.sub.0.3                                                                  15                                                                               0.4  30    10                                                                               0.4  Y.sub.2 O.sub.3                                                                      0.5                                                                              0.4  remainder                  0.5  10    ZrC.sub.0.5 N.sub.0.5                                                                  15                                                     40   0.5  20    HfC     30                                                                               0.4  30    10                                                                               0.1  MgO   0.2                                                                              0.4  remainder                                                    0.4  Y.sub.2 O.sub.3                                                                      0.3                          Prior                                                                          art                                                                            cermets                                                                        11   1.2   3    TiN     30                                                                               --   --    --                                                                               0.4  Y.sub.2 O.sub.3                                                                      3 0.4  remainder             12   1.5   2    TiC.sub.0.5 N.sub.0.5                                                                  30                                                                               --   --    --                                                                               0.1  MgO   3 0.4  remainder             13   1.5   2    TiC.sub.0.5 N.sub.0.4 O.sub.0.1                                                        30                                                                               --   --    --                                                                               0.3  CaO   3 0.4  remainder             __________________________________________________________________________

                                      TABLE 5-2                                    __________________________________________________________________________            Conditions of sintering or hot pressing                                                                Conditions of heat treatment                           Composition             Composition                                            of                 Holding                                                                             of                 Holding                             atmosphere                                                                            Pressure                                                                            Temperature                                                                           time atmosphere                                                                            Pressure                                                                            Temperature                                                                           time                                (vol %)                                                                               (Atm)                                                                               (°C.)                                                                          (Hr) (vol %)                                                                               (Atm)                                                                               (°C.)                                                                          (Hr)                         __________________________________________________________________________     Ceramics of                                                                    the invention                                                                  31     Ar:100 0.5  1600   3    Ar:100  500 1400   10                           32     Ar:100 1    1700   3    Ar:100 1000 1600   5                            33     Ar:100 1    1800   3    Ar:100 1500 1800   3                            34     Ar:90, 1    1700   5    Ar:100 1000 1500   3                                   He:10                                                                   35     N.sub.2 :100                                                                          1    1750   2    Ar:100 2000 1600   3                            36     Ar:100 2    1750   3    Ar:100 1000 1600   5                            37     Ar:100 1    1750   3    Ar:100 1000 1800   3                            38     Ar:100 0.8  1750   2    Ar:100 1500 1700   3                            39     Ar:100 1    1750   3    Ar:100 1000 1600   3                            40     Ar:100 1    1750   3    Ar:100 2000 1600   3                            Prior art                                                                      cermets                                                                        11     Ar:100 1    1700   3    --     --   --     --                           12     Ar:100 1    1750   3    --     --   --     --                           13     Ar:100 1    1750   3    --     --   --     --                           __________________________________________________________________________

                                      TABLE 6                                      __________________________________________________________________________     Composition (% by weight)                                                                               Metal                                                                              Al.sub.2 O.sub.3                                  M (CNO)          ZrO.sub.2                                                                              oxides    Av-     Theo-                               Av-          At      At  at        erage                                                                              In- retical Bending                     erage    Within                                                                             grain   grain                                                                              grain     grain                                                                              ternal                                                                             density                                                                            Hard-                                                                              strength                                                                               Cutting             grain    grains                                                                             bound-                                                                             Within                                                                             bound-                                                                             bound-    size                                                                               stress                                                                             ratio                                                                              ness                                                                               (kg/    time                size     (μm)                                                                            ary grains                                                                             ary ary Al.sub.2 O.sub.3                                                                     (μm)                                                                            (MPa)                                                                              (%) (H.sub.R A)                                                                        mm.sup.2)                                                                           FT (min)               __________________________________________________________________________     Ceramics                                                                       of the                                                                         invention                                                                      31   0.6 0.5 4.5 2   8   --  remainder                                                                            0.9 95  99.8                                                                               93.6                                                                               112  6.1                                                                               18                  32   0.6 1   29  3   7   --  remainder                                                                            0.8 63  99.7                                                                               94.2                                                                               138  6.4                                                                               24                  33   0.6 8   32  0.1 0.9 --  remainder                                                                            0.9 72  99.6                                                                               94.3                                                                               130  5.8                                                                               20                  34   0.6 1   19  6   19  --  remainder                                                                            0.6 103 99.7                                                                               94.0                                                                               112  6.0                                                                               16                  35   0.5 5   25  3   7   --  remainder                                                                            0.8 34  99.9                                                                               94.2                                                                               126  6.2                                                                               24                  36   0.5 0.5 29.5                                                                               2   8   --  remainder                                                                            0.8 82  99.8                                                                               94.1                                                                               132  6.3                                                                               20                  37   0.4 3   27  2   8   --  remainder                                                                            0.9 13  99.9                                                                               94.2                                                                               135  6.4                                                                               19                  38   0.6 2   28  2   8   --  remainder                                                                            0.9 105 99.7                                                                               94.0                                                                               126  6.4                                                                               23                  39   0.6 4   26  2   8   --  remainder                                                                            0.9 87  99.6                                                                               94.2                                                                               130  6.2                                                                               21                  40   0.6 3   27  2   8   --  remainder                                                                            0.7  3  99.7                                                                               94.1                                                                               125  6.0                                                                               20                  Prior                                                                          art                                                                            cermets                                                                        11   1.3 --  30  --  --  4   remainder                                                                            1.6  4  99.7                                                                               94.0                                                                                82  5.0                                                                               2.sup.(1)           12   1.8 --  30  --  --  3   remainder                                                                            2.3 18  99.9                                                                               94.1                                                                                90  5.2                                                                               3.sup.(2)           13   1.7 --  30  --  --  4   remainder                                                                            2.0 31  99.8                                                                               93.9                                                                                88  5.1                                                                               2.sup.(3)           __________________________________________________________________________      FT denotes "Fracture toughness                                                 .sup.(1),(3) chipped in 2 minutes                                              .sup.(2) fractured in 3 minutes                                          

                  TABLE 7-1a                                                       ______________________________________                                         Coated                                                                         ceramics of                                                                             Blending composition of substrate                                     the invention                                                                           TiCN        MgO    ZrO.sub.2                                                                            Y.sub.2 O.sub.3                                                                     Al.sub.2 O.sub.3                        ______________________________________                                         1        TiC.sub.0.5 N.sub.0.5                                                                    20    0.5  --    --   remainder                             2        TiC.sub.0.7 N.sub.0.3                                                                    30    0.5  --    --   remainder                             3        TiC.sub.0.5 N.sub.0.5                                                                    30    --   --    0.5  remainder                             4        TiC.sub.0.5 N.sub.0.5                                                                    30    --   --    0.5  remainder                             5        TiC.sub.0.5 N.sub.0.5                                                                    30    --   --    0.5  remainder                             6        TiC.sub.0.5 N.sub.0.5                                                                    30    0.5  2     --   remainder                             7        TiC.sub.0.5 N.sub.0.5                                                                    30    --   --    0.5  remainder                             8        TiC.sub.0.5 N.sub.0.5                                                                    30    0.5  --    0.5  remainder                             9        TiC.sub.0.3 N.sub.0.7                                                                    30    0.5  --    0.5  remainder                             10       TiC.sub.0.5 N.sub.0.5                                                                    30    0.5  --    0.5  remainder                             ______________________________________                                    

                                      TABLE 7-1b                                   __________________________________________________________________________            Hard coating                                                                   1st layer     2nd layer     3rd layer     4th layer                                      Av-           Av-           Av-           Av-                             Average                                                                             erage    Average                                                                             erage    Average                                                                             erage    Average                                                                             erage               Coated      layer                                                                               grain    layer                                                                               grain    layer                                                                               grain    layer                                                                               grain               ceramics of                                                                           Com- thickness                                                                           size                                                                               Com- thickness                                                                           size                                                                               Com- thickness                                                                           size                                                                               Com- thickness                                                                           size                the invention                                                                         position                                                                            (μm)                                                                             (Å)                                                                            position                                                                            (μm)                                                                             (Å)                                                                            position                                                                            (μm)                                                                             (Å)                                                                            position                                                                            (μm)                                                                             (Å)             __________________________________________________________________________     1      TiCN 0.5  200 TiN  1.5  200 --   --   --  --   --   --                  2      TiCN 0.5  200 TiN  1.5  200 --   --   --  --   --   --                  3      TiN  2.0  300 --   --   --  --   --   --  --   --   --                  4      TiC  2.0  200 TiCN 1.0  200 TiN  2.0  250 --   --   --                  5      TiN  0.5  200 TiC  1.5  200 TiCN 1.5  200 TiN  1.5  200                 6      TiC  1.0  200 TiCN 2.0  200 TiN  3.0  200 --   --   --                  7      TiC  5.0  150 --   --   --  --   --   --  --   --   --                  8      TiCN 2.0  150 TiC  2.0  150 --   --   --  --   --   --                  9      TiCN 3.0  150 --   --   --  --   --   --  --   --   --                  10     TiCN 1.0  200 TiN  2.0  200 --   --   --  --   --   --                  __________________________________________________________________________

                  TABLE 7-2a                                                       ______________________________________                                         Comparative                                                                              Blending composition of substrate                                    coated ceramics                                                                          TiCN        MgO     Y.sub.2 O.sub.3                                                                      Al.sub.2 O.sub.3                           ______________________________________                                         1         TiC.sub.0.5 N.sub.0.5                                                                    30    0.5   --    remainder                                2         TiC.sub.0.5 N.sub.0.5                                                                    30    0.5   --    remainder                                3         TiC.sub.0.5 N.sub.0.5                                                                    30    0.5   --    remainder                                4         TiC.sub.0.5 N.sub.0.5                                                                    30    0.5   --    remainder                                5         TiC.sub.0.5 N.sub.0.5                                                                    30    0.5   --    remainder                                6         TiC.sub.0.5 N.sub.0.5                                                                    30    --    0.5   remainder                                7         TiC.sub.0.5 N.sub.0.5                                                                    30    --    0.5   remainder                                8         TiC.sub.0.5 N.sub.0.5                                                                    30    --    0.5   remainder                                9         TiC.sub.0.5 N.sub.0.5                                                                    30    --    0.5   remainder                                10        TiC.sub.0.3 N.sub.0.7                                                                    40    0.5   0.5   remainder                                ______________________________________                                    

                                      TABLE 7-2b                                   __________________________________________________________________________            Hard coating                                                                   1st layer     2nd layer     3rd layer     4th layer                                      Av-           Av-           Av-           Av-                             Average                                                                             erage    Average                                                                             erage    Average                                                                             erage    Average                                                                             erage               Comparative layer                                                                               grain    layer                                                                               grain    layer                                                                               grain    layer                                                                               grain               coated Com- thickness                                                                           size                                                                               Com- thickness                                                                           size                                                                               Com- thickness                                                                           size                                                                               Com- thickness                                                                           size                ceramics                                                                              position                                                                            (μm)                                                                             (Å)                                                                            position                                                                            (μm)                                                                             (Å)                                                                            position                                                                            (μm)                                                                             (Å)                                                                            position                                                                            (μm)                                                                             (Å)             __________________________________________________________________________     1      TiCN 0.5  400 TiN  1.5  400 --   --   --  --   --   --                  2      TiCN 0.5  450 TiN  1.5  450 --   --   --  --   --   --                  3      TiN  2.0  500 --   --   --  --   --   --  --   --   --                  4      TiC  2.0  400 TiCN 1.0  400 TiN  2.0  400 --   --   --                  5      TiN  0.5  400 TiC  1.5  400 TiCN 1.5  400 TiN  1.5  450                 6      TiC  1.0  500 TiCN 2.0  400 TiN  3.0  500 --   --   --                  7      TiC  5.0  400 --   --   --  --   --   --  --   --   --                  8      TiCN 2.0  500 TiC  2.0  500 --   --   --  --   --   --                  9      TiCN 3.0  500 --   --   --  --   --   --  --   --   --                  10     TiCN 1.0  400 TiN  2.0  400 --   --   --  --   --   --                  __________________________________________________________________________

                  TABLE 8                                                          ______________________________________                                                Interrupted   Continuous                                                       cutting tests cutting tests                                                    Flank wear                                                                             Face wear Flank wear                                                                               Face wear                                          width (mm)                                                                             depth (μm)                                                                            width (mm)                                                                               depth (μm)                               ______________________________________                                         Coated                                                                         ceramics of                                                                    the invention                                                                  1        0.23      52        0.26    60                                        2        0.20      49        0.23    56                                        3        0.21      50        0.24    54                                        4        0.18      41        0.21    45                                        5        0.19      43        0.21    48                                        6        0.18      40        0.20    44                                        7        0.20      45        0.21    49                                        8        0.24      54        0.25    60                                        9        0.22      56        0.26    62                                        10       0.23      55        0.26    59                                        Comparative                                                                    coated                                                                         cermets                                                                        1        chipped in 12 minutes                                                                          chipped in 7 minutes                                  2        chipped in 11 minutes                                                                          chipped in 8 minutes                                  3        chipped in 13 minutes                                                                          chipped in 7 minutes                                  4        chipped in 12 minutes                                                                          chipped in 6 minutes                                  5        chipped in 11 minutes                                                                          chipped in 8 minutes                                  6        chipped in 11 minutes                                                                          chipped in 9 minutes                                  7        chipped in 13 minutes                                                                          chipped in 8 minutes                                  8        chipped in  9 minutes                                                                          chipped in 8 minutes                                  9        chipped in 10 minutes                                                                          chipped in 7 minutes                                  10       chipped in 13 minutes                                                                          chipped in 7 minutes                                  ______________________________________                                     

What is claimed is:
 1. An Al₂ O₃ -based ceramic consisting essentially of:5% to 50% by weight of a hard dispersed phase composed of at least one compound which is selected from the group consisting of metal carbide, metal nitride, metal carbonitride, metal oxy-carbide, metal oxynitride and metal carbo-oxynitride, and is represented by M(CNO), wherein M is at least one metal selected from the group consisting of Ti, Zr and Hf; no greater than 25% by weight of ZrO₂ ; and balance Al₂ O₃ matrix containing an additive dissolved in the Al₂ O₃ grains so as to form a solid solution therewith, said additive being contained in an amount of 0.01% to 12% by weight with respect to the Al₂ O₃ matrix and being composed of at least one oxide of an element selected from the group consisting of Y, Mg, Cr, Ni, Co and rare earth elements; said Al₂ O₃ grains having an average grain size of no larger than 1.0 μm while said hard phase constituent has an average grain size of no larger than 0.6 μm, wherein said hard phase constituent is present at the grain boundary of and within the Al₂ O₃ grains and the resulting Al₂ O₃ -based ceramic having a three-point bending strength of at least 112 kg/mm².
 2. An Al₂ O₃ based ceramic according to claim 1, wherein said compound M(CNO) of said hard dispersed phase is the one represented by M(C_(x) N_(y) O_(z)), wherein 0.95≦x+y+z≦1, 5/95≦x/y≦70/30 and 0≦z≦0.1.
 3. An Al₂ O₃ based ceramic according to claim 2, wherein residual compressive stress exerted on said Al₂ O₃ matrix is no less than 20 MPa.
 4. An Al₂ O₃ based ceramic according to claim 2, wherein said hard phase constituent is composed of at least one compound selected from the group consisting of titanium carbide, titanium nitride and titanium carbo-nitride, and is represented by Ti(C_(x) N_(y)), wherein 5/95≦x/y≦70/30.
 5. An Al₂ O₃ based ceramic according to claim 2, further comprising a hard coating vapor-deposited thereon, said hard coating having a thickness of 0.3 to 30 μm and being comprised of one or more layers of at least one composition selected from the group consisting of carbide, nitride, carbo-nitride, oxy-carbide of titanium, and Al₂ O₃.
 6. An Al₂ O₃ based ceramic according to claim 4, further comprising a hard coating vapor-deposited thereon, said hard coating having a thickness of 0.3 to 30 μm and being comprised of one or more layers of at least one composition selected from the group consisting of carbide, nitride, carbo-nitride, oxy-carbide of titanium, and Al₂ O₃.
 7. An Al₂ O₃ based ceramic according to claim 1, wherein said additive is dissolved in said Al₂ O₃ grains so as to form a solid solution herewith.
 8. An Al₂ O₃ -based ceramic consisting of:5% to 50% by weight of a hard dispersed phase composed of at least one compound which is selected from the group consisting of metal carbide, metal nitride, metal carbonitride, metal oxy-carbide, metal oxynitride and metal carbo-oxynitride, and is represented by M(CNO), wherein M is at least one metal selected from the group consisting of Ti, Zr and Hf; no greater than 25% by weight of ZrO₂ ; and balance Al₂ O₃ matrix containing an additive dissolved in the Al₂ O₃ grains so as to form a solid solution therewith, said additive being contained in an amount of 0.01% to 12% by weight with respect to the Al₂ O₃ matrix and being composed of at least one oxide of an element selected from the group consisting of Y, Mg, Cr, Ni, Co and rare earth elements; said Al₂ O₃ grains having an average grain size of no larger than 1.0 μm while said hard phase constituent has an average grain size of no larger than 0.6 μm, wherein said hard phase constituent is present at the grain boundary of and within the Al₂ O₃ grains and the resulting Al₂ O₃ -based ceramic having a three-point bending strength of at least 112 kg/mm².
 9. An aluminum based ceramic as defined in claim 1, wherein the ZrO₂ is present in an amount of 1 to 25% by weight.
 10. An Al₂ O₃ -based ceramic consisting of:5% to 50% by weight of a hard dispersed phase composed of at least one compound which is selected from the group consisting of metal carbide, metal nitride, metal carbonitride, metal oxycarbide, metal oxynitride and metal carbo-oxynitride, and is represented by M(CNO), wherein M is at least one metal selected from the group consisting of Ti, Zr and Hf; 1 to 25% by weight of ZrO₂ ; and balance Al₂ O₃ matrix containing an additive dissolved in the Al₂ O₃ grains so as to form a solid solution therewith, said additive being contained in an amount of 0.01% to 12% by weight with respect to the Al₂ O₃ matrix and being composed of at least one oxide of an element selected from the group consisting of Y, Mg, Cr, Ni, Co and rare earth elements; said Al₂ O₃ grains having an average grain size of no larger than 1.0 μm while said hard phase constituent has an average grain size of no larger than 0.6 μm, wherein said hard phase constituent is present at the grain boundary of and within the Al₂ O₃ grains and the resulting Al₂ O₃ -based ceramic having a three-point bending strength of at least 112 kg/mm². 